1. Field of the Invention
This invention relates to improvements in a musical scale generating circuit used in melody generators of the electronic timepiece, and more particularly to a musical scale generating circuit capable of obtaining a desired scale frequency (especially one associated with a frequency dividing ratio that is odd number) from a low-frequency source.
2. Prior Art
A conventional timepiece melody generator of this type is disclosed in the specification of Japanese Patent Application Laid-Open No. 56-133674.
FIG. 35 is a block diagram illustrating this conventional melody generator.
The generator includes a programmable frequency divider 4 for dividing a high-frequency signal from an oscillator 2 on the basis of frequency dividing ratio data from a ROM 6 storing frequency dividing ratios in a sequence corresponding to a melody, and for applying the resulting signal to a tone generating circuit 8 which proceeds to generate a tone of a desired scale.
The frequency dividing ratio data stored in the ROM 6 are decided in the following manner.
If the melody is, for example, the same as that produced by the clock at Westminster Abbey, the scales used to generate it are of four types, namely C.sub.5 # (about 555 Hz), B.sub.4 (about 496 Hz), A.sub.4 (about 443 Hz) and E.sub.4 (about 331 Hz). By combining these four scales, four melodies indicating an on-the-hour time, quarter past, half past and quarter of the hour are produced, and the "Westminster melody" is constituted by these four melodies.
The sequence through which the scales of the on-the-hour, quarter-past, half-past and quarter-of melodies are combined is as illustrated in Table I.
TABLE I __________________________________________________________________________ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 __________________________________________________________________________ ON THE HOUR A.sub.4 C.sub.5 # B.sub.4 E.sub.4 A.sub.4 B.sub.4 C.sub.5 # A.sub.4 C.sub.5 # A.sub.4 B.sub.4 E.sub.4 E.sub.4 B.sub.4 C.sub.5 # A.sub.4 15 MIN AFTER C.sub.5 # B.sub.4 A.sub.4 E.sub.4 30 MIN AFTER A.sub.4 C.sub.5 # B.sub.4 E.sub.4 A.sub.4 B.sub.4 C.sub.5 # A.sub.4 45 MIN AFTER C.sub.5 # A.sub.4 B.sub.4 E.sub.4 E.sub.4 B.sub.4 C.sub.5 # A.sub.4 C.sub.5 # B.sub.4 A.sub.4 E.sub.4 __________________________________________________________________________
If scale frequencies corresponding to the respective scales are outputted by the programmable frequency divider 4 in the order of the scales shown in Table I, the tone generating circuit 8 will generate the "Westminster melody".
Accordingly, if the frequency dividing ratios of the programmable frequency divider 4 for outputting the scale frequencies are made to correspond to respective ones of these scales and these frequency dividing ratios are set and stored in the ROM 6 in order in place of the scales of Table I, then the programmable frequency divider 4 will sucessively output scale frequency signals conforming to the respective scales of the melody.
It is assumed that the oscillator 2 employs a quartz oscillator and that the programmable frequency divider 4 used in the generator of FIG. 35 has a seven-bit preset input and is capable of dividing by a maximum of 128. Table II shows the corresponding scales, scale frequencies and frequency dividing ratios in such case.
TABLE II ______________________________________ Oscillator Frequency Scale Scale Frequency Dividing Ratio ______________________________________ 32,768 Hz C.sub.5 # 555.390 Hz 59 B.sub.4 496.485 Hz 66 A.sub.4 442.811 Hz 74 E.sub.4 330.990 Hz 99 ______________________________________
As shown in Table II, a scale frequency corresponding to each scale can be obtained by dividing the output signal of the oscillator 2 at the respective frequency dividing ratio.
The ROM 6 practically stores the frequency dividing data which is the complement, in the form of a binary number, of each frequency dividing ratio, namely 69 (128-59) for the frequency dividing ratio 59, 62 (128-66) for the frequency dividing ratio 66, etc., as shown in Table III. These complementary numbers are successively applied to the programmable frequency divider 4 as the frequency dividing data.
TABLE III ______________________________________ Scale Binary Number ______________________________________ C.sub.5 # 1000101 complement of 59 B.sub.4 0111110 complement of 66 E.sub.4 0110110 complement of 74 A.sub.4 0011101 complement of 99 ______________________________________
When these frequency dividing ratio data are fed into the programmable frequency divider 4, the latter is forcibly preset and starts counting from the numerical values (69, 62, 54, 29) until the final stage. The end result is that the signal from the oscillator 2 is frequency-divided at the frequency dividing ratios 59, 66, 74, 99.
When the programmable frequency divider 4 is forcibly preset in mid-course as in the prior art described above, the scale frequency signals obtained as a result are as depicted in FIG. 36. The signals do not exhibit a duty cycle of 50% and contain various high-frequency components. The resulting problem is that the tone generating circuit 8 does not generate a clear tone.
Measures have been considered for solving the foregoing problem.
Specifically, as shown in FIG. 37, a 1/2 frequency divider 10 is provided at the output of the programmable frequency divider 4 shown in FIG. 35 in order to frequency-divide the output of the programmable frequency divider 4 by two. Though the duty cycles can be made 50% by providing the 1/2 frequency divider 10, the frequency dividing ratios of the programmable frequency divider 4 in such case must be set beforehand to one-half those indicated in Table II. This means that the frequency dividing ratios 66 (B.sub.4), 74 (A.sub.4) need only be set to 33, 37, respectively. However, when performing frequency division at the ratios 59 (C.sub.5 #), 99 (E.sub.4), frequency dividing ratios that are one-half these values cannot be produced. Therefore, it is necessary that the output frequency of the oscillator be increased twofold (to 64 KHz).
Since a source of such high-frequency oscillation employs a large number of cascaded flip-flops, the oscillator is higher in cost and consumes a greater amount of power.